Réf. Bouwer & al. 2008 - A

Référence bibliographique complète

BOUWER, L.M., VERMAAT, J.E., AERTS, J.C.J.H. 2008. Regional sensitivities of mean and peak river discharge to climate variability in Europe. Journal of Geophysical Research, Vol. 113, D19103, doi:10.1029/2008JD010301.

Abstract: Variability of atmospheric circulation is thought to be the most important factor causing annual and decadal variability of fresh water fluxes from the continents. Previous studies, however, have rarely established the sensitivity of the basins to atmospheric circulation variability. Here we present an analysis of long-term (>30 years) links between atmospheric forcing and winter (December–February) precipitation, and sensitivities of annual mean and maximum winter discharges observed at up to 608 stations across Europe. Links to four atmospheric indices are examined: the North Atlantic Oscillation (NAO) index, the Arctic Oscillation (AO) index, the frequency of west circulation (FWC) as described by the subjective Großwetterlagen classification, and the north to south sea level pressure difference across the European continent (SLPD). The results show that annual maximum discharges are more sensitive to variability of atmospheric circulation than mean discharges. Mean discharges vary on average between 8 and 44%, while peak discharges vary between 10 and 54% per unit index change. Discharges in Iberia and Scandinavia are more sensitive than those in central and northwest Europe. Discharge closely follows variability of atmospheric circulation. Compared with FWC and SLPD, the NAO and AO indices have only limited use for analyzing climate impacts in river basins in northwest Europe.

Mots-clés

 

 

Organismes / Contact

• Institute for Environmental Studies, Faculty of Earth and Life Sciences, Vrije Universiteit, Amsterdam, Netherlands.

 

(1) - Paramètre(s) atmosphérique(s) modifié(s)

(2) - Elément(s) du milieu impacté(s)

(3) - Type(s) d'aléa impacté(s)

(3) - Sous-type(s) d'aléa

 

 

 

 

 

Pays / Zone

Massif / Secteur

Site(s) d'étude

Exposition

Altitude

Période(s) d'observation

 

 

 

 

 

 

 

(1) - Modifications des paramètres atmosphériques

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

 

(2) - Effets du changement climatique sur le milieu naturel

Reconstitutions

 

Observations

Significant correlation is found between atmospheric circulation variability, described by the NAO, AO, FWC and SLPD indices, and peak and mean river discharges. West circulation over Europe is causing increased discharges in most basins. River basins have different sensitivities to atmospheric circulation variability across Europe, depending on position on the continent, local climate, vegetation and other basin conditions. Annual maximum discharges appear to be more sensitive to variability of atmospheric circulation that mean discharges, as the regression coefficients (β1) are higher per unit change of the atmospheric circulation indices (for nine out of twelve cases). The authors had only limited access to long-term data series from south Europe, leading to gaps in our analysis of sensitivities for this area. Compared with FWC and SLPD, the NAO and AO indices have only limited use for analyzing climate impacts in river basins in northwest Europe.

Over time, there is great variability of mean and peak discharges, as shown by the analysis of principal components of river discharges for different regions. Although some periods, in particular the 1990s, stand out in terms of high mean and peak discharge occurrence in northwest Europe, there are not necessarily trends toward higher discharges, as periods of high discharge have also occurred earlier in the record, in particular during the 1910s and 1920s. Discharges follow closely the variability of atmospheric circulation. This underscores the difficulty to determine trends in discharge records, as the findings will depend on the time span that is being studied [Woo et al., 2006], the river basins that are selected [Milly et al., 2002], and as flood events may be clustered in certain years and decades [Blöschl et al., 2007]. The most comprehensive trend analysis available, which used some of the longest series in the GRDC data set, detected both upward and downward trends for annual maximum discharges across Europe, and found no systematic trends toward more extreme discharges [Kundzewicz et al., 2005]. Mudelsee et al. [2003] found a decrease in recent extreme discharges for extended discharge records for the Elbe and Oder rivers in northwest Europe.

This research indicates that a sustained or increased strength and frequency of positive phases of the NAO and AO indices and west atmospheric circulation (FWC and SLPD) over northwest Europe implies considerable increases in peak discharges in northern Europe and declines in water availability in south Europe in winter. The estimation of future impacts on discharge variability and change will depend on the ability of general circulation models (GCMs) to capture such circulation variability. Analysis of GCM output suggests that winters in Europe may see more frequent west circulation and increased precipitation [Van Ulden and van Oldenborgh, 2006]. James [2006] showed that it is also possible to link the output of GCMs to the subjective Großwetterlagen classification system, in order to estimate such changes. But it appears that there are systematic problems with the simulation of historic circulation changes, as current GCMs are not able to capture the recent tendency toward more frequent west circulation [Gillett, 2005]. Indeed, atmospheric circulation biases may be great with particular large impacts on precipitation, and responses vary between GCMs, especially in summer [Van Ulden and van Oldenborgh, 2006]. There is clearly a need to improve the skill of the circulation models to accurately capture the variability of atmospheric circulation, as this skill will improve our understanding of potential atmospheric impacts on river discharges and flood events.

Modélisations

 

Hypothèses

 

 

Sensibilité du milieu à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

Climate variability and climate change may modify the availability of fresh water, as well as the frequency and intensity of flood events. In particular west atmospheric circulation, consisting of zonal flow, is known to determine fluctuations in wintertime precipitation in west Europe [Hurrell, 1995]. Many studies, some of which are listed in the present study, have assessed whether river discharge in Europe is closely connected to atmospheric circulation variability. Such studies are important in order to estimate the impacts of current climate variability, impacts of anthropogenic climate change on the global water cycle [Zhang et al., 2007], and the potential impacts of future climate change as projected by general circulation models. However, previous studies have rarely established the sensitivity of river basin discharge to atmospheric circulation variability. This paper aims to fill that gap by systematically studying the sensitivity of European river basins.

This paper presents a systematic analysis of the spatial patterns of connections between circulation patterns and precipitation, as well as connections between circulation patterns and river discharge in Europe. The analysis helps to understand historic variability and predict changes in future river discharges. The paper provides year-to-year correlations between four atmospheric forcing indices and precipitation, as well as between these four indices and mean and peak river discharges in winter. This is done for entire river discharge records that are readily available. Additionally, we quantify the sensitivity of the river discharges to atmospheric forcing. The forcing indicators that are assessed are the NAO index; the Arctic Oscillation (AO) index, the frequency of west circulation (FWC) as described by the subjective Großwetterlagen classification system, and the normalized north to south sea level pressure difference (SLPD) over Europe, which is an objective measure of west atmospheric flow across the European continent.

Data on daily river discharges have been collected for 586 gauging stations across Europe, complemented by monthly discharge records for 49 of these stations that comprised longer periods, and 22 separate monthly records. The discharge data were obtained from the Global Runoff Data Centre (GRDC) in Koblenz, Germany. These records include many of the longest discharge records for the major rivers in Europe. The monthly and daily data were combined in order to arrive at a data set of 608 seasonal mean discharge records and 586 records for annual maximum discharge series for individual stations. There is a bias of long data sets toward locations in northern European countries. For these countries also more discharge records are available, often at different locations along the same river, or its tributaries.

Data on monthly precipitation were collected from the CRU TS 2.0 gridded data set [Mitchell and Jones, 2005]. From these data the total winter precipitation was derived for December–February over the period 1901–1999 on a 0.5 by 0.5° grid.

The NAO index has been constructed by Hurrell [1995], consisting of the difference of normalized sea level pressures anomalies between Ponta Delgada in the Azores and Stykkisholmur/Reykjavik in Iceland. The data on the Arctic Oscillation (AO) index is based on the normalized principal component values for the period December–March for the Northern Hemisphere (20N–90N). Data on the NAO (1864–2002) index for December–February and the AO (1898–2005) index for December–March were retrieved from the website http://www.cgd.ucar.edu/cas/jhurrell/indices.html.

The German Großwetterlagen classification system provides information on the daily type of atmospheric circulation over Europe. This subjective classification system is based on the locations of high and low pressures and ridges and troughs at the 500 hPa level. From Bouwer et al. [2006] it follows that winter river discharge in many large basins in Europe is significantly correlated to the frequency of west circulation as described by this classification. Data for the period 1881–2004 was collected from the catalog produced by Gerstengarbe and Werner [2005], updated with data from the German Weather Service up to 2005. A data set of the annual frequency (number of days) of west circulation in winter was constructed, using the classes west anticyclonic (WA), west cyclonic (WZ), southwest (WS) and angular west (WW). From this annual frequency, a normalized index was constructed in a similar way as the NAO index.

An index of normalized sea level pressure differences (SLPD) between the points at 60°N, 0°E, and 35°N, 5°W for the period 1850–2006 was constructed from the data compiled by Allan and Ansell [2006]. Data for two grid cells centered on these two points for the period 1850–2006 was retrieved from the website http://climexp.knmi.nl. The construction of the normalized SLPD index on the basis of the sea level pressure data is similar to the construction of the NAO index.

The sensitivity of mean and peak river discharges to atmospheric forcing is estimated using a simple linear least squares regression function [see details in the study].

 

(3) - Effets du changement climatique sur l'aléa

Reconstitutions

 

Observations

 

Modélisations

 

Hypothèses

 

 

Paramètre de l'aléa

Sensibilité des paramètres de l'aléa à des paramètres climatiques

Informations complémentaires (données utilisées, méthode, scénarios, etc.)

 

Variation in west atmospheric circulation is often expressed by the North Atlantic Oscillation (NAO) index. Atmospheric circulation as described by the NAO index, is linked most closely to precipitation variability in winter [Hurrell, 1995], as well as to winter river discharges [Dettinger and Diaz, 2000; Trigo et al., 2004]. The NAO index and other atmospheric indicators are based on the north-south pressure gradients over the northern hemisphere of the planet. The north-south pressure gradient gives an indication of the frequency, strength and location of western atmospheric flow over the European continent. During periods with a small pressure gradient, low atmospheric pressure weather systems on the Atlantic travel in the direction of the Mediterranean causing increased rainfall, while during phases of a high pressure gradient low-pressure systems travel toward Scandinavia and precipitation in northern Europe is increased. Additionally, prolonged west circulation may substantially reduce evaporation through increased cloudiness and increased air humidity and increase soil humidity, potentially resulting in increased river discharges and increased peak flows. River discharge in Europe during the winter months (December, January and February) represents between 23 and 43% of total annual runoff in the major European river basins [Bouwer et al., 2006]. Most major peak discharge and flood events have occurred in Europe during winter in response to prolonged west atmospheric circulation and associated precipitation [Caspary, 1995; Tu et al., 2005a], while during the summer season flooding may occur owing to other weather patterns [Becker and Gru¨newald, 2003; Mudelsee et al., 2004].

 

 

(4) - Remarques générales

 

 

(5) - Syntèses et préconisations

 

Références citées :